Without "the" it means in general and not directed at any particular traffic. Traffic is slow because careless drivers causing accidents. Traffic at night is usually non-existent unless there are closures for work or some major accident. The traffic today is terrible because the President is coming to town. The traffic in Los Angeles is always a problem. -- Here, "the" is for emphasis and not pertain to any particular traffic. You can also say, "Traffic in Los..." This means in general.

"The"is not necessary in that sentence or context. Here are some examples taken directly from my city's website today: For months, construction on busy Texas 26 in Colleyville has slowed traffic. We spent a lot of time in our car, stuck in traffic. All lanes of Interstate 35E in Carrollton reopened to traffic Wednesday afternoon. The massive 16-lane highway travels through Bedford, Euless, Fort Worth, Irving and Dallas and will hopefully bring some traffic relief. Traffic is being forced onto the frontage roads as crews demolish the Abram Street exit. A 5 car pileup has caused traffic to slow and has created a 5 mile backup.

"the traffic is pretty slow" is better other examples are Traffic in London is very bad. The traffic on Sundays is the worst. I never leave before 7:30pm on a Friday because I know the traffic will be terrible.